1. Field of the Invention
The invention relates to a switched mode power supply comprising a start-up device for supplying electrical energy to the controller in a start-up phase. Further, the invention relates to a method for starting up such a switched mode power supply, a controller start-up device for use in such a switched mode power supply, and a display apparatus comprising such a switched mode power supply.
Switched mode power supplies are used in a wide range of electronic equipment. Examples of such electronic equipment include computing equipment, television and video equipment, as well as portable telecommunication devices. Switched mode power supplies convert a DC primary voltage, such as a battery voltage or a rectified AC line voltage, into one or more secondary voltages.
2. Description of the Related Art
U.S. Pat. No. 5,581,453 discloses a start-up circuit for a switched mode power supply. The power supply comprises a transformer which is switched by a switching transistor to be periodically connected to an input voltage. Periodic pulses for driving the switching transistor are provided by a power supply control circuit. To be able to start such a power supply, the power supply is equipped with a start-up circuit comprising a start-up resistor and a capacitor. The start-up circuit provides the power supply control circuit with initial electrical energy. In addition thereto, the power supply control circuit is supplied with energy from an additional winding on the transformer, the voltage on this winding being rectified and supplied to the power supply control circuit. During start-up of the power supply, the supply of the power supply control circuit is fully accounted for by the start-up circuit. The start-up resistor will charge the capacitor, causing the supply voltage of the power supply control circuit to rise. Initially, the voltage on the transformer is low and, consequently, the additional winding of the transformer does not contribute to the supply of the power supply control circuit. After a certain start-up time, the output voltage of the power supply starts to rise. Consequently, the voltage available on the additional winding of the transformer starts to rise. When the voltage on the additional winding is sufficiently large, the winding will take over the supply of the power supply control circuit from the start-up circuit.
Typically, the start-up resistor is dimensioned to provide only a current which is sufficient to start up the power supply control circuit. The normal operating current of the power supply control circuit is much higher. Although not disclosed in the above publication, this causes the arrangement to have a built-in overload protection. In case the output voltage of the power supply lowers, for example, because of an overload or a short circuit, the voltage supplied by the transformer will decrease and consequently, the voltage on the additional winding will decrease. This causes the supply voltage of the power supply control circuit to drop as the start-up resistor has been dimensioned such as to only provide a small start-up current, which is insufficient to power the power supply control circuit during normal operation. As a result, the switching will stop and the power supply will enter a start-up sequence, attempting a new start up.
A disadvantage of this power supply is that an additional winding in the transformer is needed. This complicates the design of the transformer. On the one hand, this winding needs to be magnetically well coupled to the secondary side of the transformer, thus it needs to be wound physically close to the secondary winding. On the other hand, the additional winding is connected to the primary side of the transformer. Consequently, the additional winding needs to be thoroughly insulated, to avoid safety problems with the insulation between the primary side and secondary side of the transformer. In general, a triple insulated wire is used. This makes the transformer relatively expensive, complex and difficult to manufacture.
Another disadvantage is that the start-up time is long. To reduce dissipation in the start-up resistor, the value of the start-up resistor is chosen to be as high as possible, however, at the cost of a slow start-up, because the high value of the resistor results in only a small start-up current available for charging the capacitor coupled therewith.
Related to the above disadvantage is another disadvantage, being that the start-up resistor will continue to dissipate electrical energy during the operation of the power supply. As power efficiency is a premium requirement in power supplies, this continuous power dissipation is undesirable. To make the power supply suitable for a wide input supply voltage range, the value of the start-up resistor has to be chosen low, so as to guarantee sufficient start-up current for the power supply control circuit during start-up at a minimum input supply voltage. However, this low value will cause a too large current through the resistor at high input voltages, and thus causes a large dissipation in the resistor. As explained above, also the requirements for a quick start-up of the power supply will tend to decrease the value of the start-up resistor, thus also increasing power losses.
In an attempt to overcome the disadvantage related to the power dissipation, U.S. Pat. No. 5,581,453 discloses a switching transistor, connected with its collector-emitter main current path in series with the start-up resistor. During start-up of the power supply, the switching transistor is in a conducting state, causing the resistor to supply a start-up current. During normal operation of the power supply, the switching transistor is in a non-conducting state, stopping the dissipation in the start-up resistor. A disadvantage of this set-up is that it easily leads to degradation of the switching transistor, as the switching transistor stops conducting by a reverse bias on the base-emitter junction. As is known, in regular transistor designs, only a few volts base-emitter reverse bias will irreversibly degrade the transistor by causing base-emitter breakdown. This condition can easily occur, for example, by a dip in the input voltage or by a raise of the voltage supplied by the additional winding, e.g., caused by load variances or manufacturing tolerances in producing the transformer. Also, it should be clear that this set-up, although attempting to solve the power dissipation problem, will not solve the above-mentioned problem associated with the third winding in the transformer.
An object of the invention is to remove, or at least reduce, the problems associated with the prior art. Another object of the invention is to simplify the design of a switched mode power supply.
A first aspect of the invention provides a switched mode power supply for conversion of an input voltage into at least one output voltage, comprising an inductive device for transforming the input voltage into the at least one output voltage; a switching device for periodically coupling the inductive device to the input voltage; a controller for controlling the switching of the switching device, and a start-up device for supplying electrical energy to the controller in a start-up phase, characterized in that the switched mode power supply further comprises a bypass-device coupled in parallel to the start-up device for bypassing the start-up device when the switched mode power supply is in regulation.
A second aspect of the invention provides a method of starting up a switched mode power supply comprising the steps controlling a start-up device to supply electrical energy to a switching controller of the switched mode power supply; and bypassing the start-up device when the switched mode power supply is in regulation.
A third aspect of the invention provides a controller start-up device for use in a switched mode power supply apparatus, the controller start-up device comprising an input port and an output port; and a start-up device coupling the input port to the output port, characterized in that the controller start-up device comprises a bypass-switch connecting the input port to the output port, if the power supply is in regulation.
A fourth aspect of the invention provides a display apparatus comprising a switched mode power supply as described above in the first aspect of the invention.
By bypassing the start-up device by a bypass-device, two operating conditions can be discerned. When the bypass-device is open, the controller is supplied with a start-up current by the start-up device, enabling a start of the switching of the switching device. When the bypass-switch is closed, the limitation in the supply current caused by the start-up device is eliminated, causing the controller to have full driving power available to normally control the switching device, resulting in normal operation of the power supply. Also, when the bypass-switch is closed, a dissipation in the start-up device is eliminated, which increases the overall energy-efficiency of the power supply. As the dissipation in the start-up device during normal operation is eliminated, the start-up device can be dimensioned for optimum start-up time, thus resulting in a reduced start-up time.
In an embodiment of the invention, the bypass device is closed to bypass the start-up device when it is detected that the switched mode power supply is in a normal operation wherein the secondary voltage is stabilized. By detecting, with a detection device, an output quantity of the power supply apparatus indicating that the power supply is in regulation, such as comparing the value of a secondary voltage with a voltage reference, it is possible to operate the bypass device depending on an output quantity of the power supply apparatus, such as the value of the secondary voltage, or a current in the secondary winding, or an output power.
In another embodiment of the invention, the bypass device is closed to bypass the start-up device when it is detected that the secondary voltage exceeds a reference voltage. The switched mode power supply has a line voltage connected part comprising the primary winding, the switching device, the controller and the opto-coupler transistor which is connected to the line input voltage. The switched mode power supply has a line voltage isolated part comprising the secondary winding and the opto-coupler diode. The detected crossing of the reference voltage by the secondary voltage is transferred, via the opto-coupler diode and transistor, from the line voltage isolated part to the line voltage connected part. An opto-coupler is able to achieve excellent galvanic isolation by simple means.
Consequently, when the secondary voltage is low, such as during start-up of the power supply, the bypass device is open, and thus the controller is supplied with a start-up current only. If this start-up current is able to successfully start-up the power supply, then the secondary voltage will rise, and the detection device detects that the output quantity indicates that the power supply is in normal operation. For example, if the secondary voltage exceeds the limit set by the detection device, then the detection device causes the bypass device to close via the control circuit, so as to maintain sufficient operating current for the controller to be able to continue normal operation.
The output quantity of the power supply apparatus may represent one or more secondary voltages, one or more output currents of the power supply apparatus, any combination of any secondary voltage and output current(s), or any other value providing suitable means for determining the operation condition of, or which determines the control of the power supply apparatus, such as a voltage generated in an auxiliary winding of the inductive device.
In another embodiment of the invention, the main current path of the opto-coupler transistor is also coupled to the controller for controlling the switching of the switching device. In this way, the opto-coupler transistor also provides input to the control device for controlling the switching of the switching device. In this manner, efficient use of components can be achieved, causing a reduction in total component cost. Also, the start-up behavior can be improved by this combined use of components, as the tolerances in these components have similar effects on the behavior of the bypass device as well as on the behavior of the switching of the controller.
In the case of an overload, the detection device will detect that the secondary voltage is low, and, consequently, the detection device will cause the bypass device to open, via the opto-coupler. This will automatically bring the power supply into start-up mode again and the power supply will attempt to restart, thus providing an electronic protection against overload.
The bypass device advantageously comprises a transistor, allowing for a simple and cost effective switching.
As the start-up device can be advantageously coupled to a primary voltage, or to a stand-by voltage supply, whichever is of a suitable value, this makes the use of an additional winding on the inductive device unnecessary, thus simplifying the design and manufacturing of the inductive device. Also, this avoids safety problems occurring particularly in the insulation of a transformer. An additional winding, with its associated isolation requirements, is not required. By avoiding the additional winding on the transformer and coupling the start-up device to a well defined voltage supply, such as a stand-by voltage supply, the start-up of the power supply as well as the restart in case of an overload are improved. The start-up does not depend any more on the magnetic coupling of an additional winding inside the transformer, and it does not depend any more on a primary voltage, which, in most cases, has a large input voltage range. This makes the start-up behavior of the power supply easier to control and predict, and the adverse influence of the tolerances of the transformer and the input voltage range can be eliminated.
Advantageously, the start-up device comprises a start-up resistor, connected between an input terminal and an output terminal of the start-up device, for providing a start-up current to the controller, and a capacitor, connected between the output terminal of the start-up device and ground. The value of the resistor can be chosen such that the resistor will supply an adequate start-up current to the controller, the value of the capacitor can be chosen such as to enable sufficient time delay to assure a sufficiently long start-up time. The combination of the resistor and capacitor is be chosen such as to assure a certain time between successive attempts to start-up the power supply in case of an overload, and to assure a reliable start-up of the power supply.
As explained above, because of the presence of the bypass device, dimensioning of the resistor for minimum power dissipation in not required, enlarging the degree of freedom when dimensioning the start-up device. This allows for choosing a low value of the resistance of the resistor such that a short start-up time is guaranteed, while avoiding a continuous high dissipation associated with such a low resistance value, because the resistor is bypassed once the start-up has taken place.
As the inductive device can comprise a transformer or an inductor, the subject invention can be used with various types of switched mode power supplies, such as a so-called fly-back, a step-up, a feed-forward or a resonant converter which are or are not line voltage isolated.